1. Field of the Invention
The present invention relates to a patient cassette for use in a modular rebreathing circuit, a modular rebreathing circuit system, and an anesthesia apparatus.
2. Description of the Prior Art
In the field of mechanical ventilation and breathing aid, there are different types of breathing circuits used to deliver a desired gas mixture to a patient. Some of these circuits are rebreathing circuits in which substantially all or a part of the gas exhaled by a patient during an exhalation is re-supplied to the patient during the following inhalation. Such rebreathing circuits are often used when expensive additive gases are administered to the patient besides the necessary life sustaining gas mixture. By re-supplying the exhaled gas to the patient, the additive gas not assimilated by the patient in previous inhalations may be absorbed during the following inhalation.
Situations in which rebreathing circuits are often used are, e.g., in treatment of severe cases of asthma wherein helium may be used as additive, in diagnostic computer tomography (CT) treatment wherein xenon may be used as contrast medium, and not least in inhalation anesthesia wherein different anesthesia gases may be used as additive to block the perception of pain and other sensations to, e.g., allow patients to undergo surgery.
There are rebreathing circuits adapted to isolate particular gas components in the exhalation gas and re-supply only the isolated components to the patient during the following inhalation. One such system is disclosed in U.S. Pat. No. 5,471,979 in which an anesthesia reflecting breathing circuit is described. During exhalation, the exhalation gas passes through an adsorption filter wherein the anesthetic that was not absorbed in the patient's lungs during the previous inhalation is adsorbed, while the majority portion of the exhalation gas passes through the filter and is evacuated from the breathing circuit. During the following inhalation, when a flow of breathing gas passes the adsorption filter from the opposite direction, the adsorbed anesthetic is desorbed from the adsorption material and re-supplied to the patient. That is, the adsorption filter operates as a reflector of anesthetics and is sometimes also referred to as an anaesthetic reflector.
One disadvantage with this type of rebreathing circuit is that the adsorption filter is unable to adsorb, and hence reuse, gases composed by small-sized molecules, such as oxygen (O2), helium, and nitrous oxide/laughing gas (N2O). Due to the inability to reuse these gases, a high flow of breathing gases is required. A high flow of breathing gases is undesirable not only because it implies high consumption of gases, it is also harmful to the environment since nitrous oxide is a greenhouse gas. Furthermore, a low breathing gas flow is desirable to better preserve tracheal heat and moisture, prevent soda lime granules in carbon dioxide absorbers from drying, and preserve patient body temperature.
Examples of prior-art breathing circuits arranged to reuse not only the anesthetic gases but the majority portion of all exhalation gases are shown in FIGS. 1A-1C. The breathing circuits 100A-C are breathing circuits of circle type, often simply referred to as circle systems, in which substantially all of the exhaled gas, after removal of carbon dioxide (CO2), is re-supplied to the patient during inhalation.
The functionality of the prior-art circle systems 100A-C will now be described with the aid of reference numerals in the drawings, which reference numerals are the same in the different drawings when denoting circuit components having a similar functionality.
The circle systems 100A-C all have a drive circuit 101A-C arranged to assist or control the breathing of a patient 102 connected to the circle system, while at the same time provide for recirculation of the exhalation gases. FIG. 1A illustrates a circle system in which the drive circuit is a mechanical ventilator 106 which is arranged to deliver a controlled flow of drive gas which acts directly on the exhalation gases from the patient 102 via a volume reflector or an exchanger 108. During exhalation, the gases exhaled by the patient 102 pass through a patient connector 104 to which the patient 102 is connected, into an expiration leg 110, and further into the volume reflector 108. The ventilator 106 then initiates inhalation of the patient 102 by opening an inspiratory valve 111 through which a drive gas flow is delivered to the volume reflector 108 via an inspiratory portion 112. The drive gas, typically oxygen, “pushes” the exhalation gases back to the patient via an inhalation leg 113 and the patient connector 104. Before the exhalation gases are re-supplied to the patient, carbon dioxide is removed by a CO2 absorber 114 and additional fresh gas, typically comprising a mix of oxygen, nitrous oxide and anesthetic, are added from a fresh gas supply line 116. During the exhalation phase, the inspiratory valve 111 is closed and an expiratory valve 117 is opened to conduct the drive gas that is pushed out of the volume reflector 108 by the exhalation gases away from the circle system 100A through an expiratory portion 118 of the ventilator 106. The expiratory portion 118 of the ventilator is typically connected to a scavenging system for isolating the discharged gas, or to a recovery system for recovering at least some of the gas components in the discharged gas. The volume reflector 108 should preferably be designed so that a well-defined front is formed between the drive gas and the exhalation gases so that a minimum of mixing occurs between the gas volumes. During ventilation, this front between the drive gas and the exhalation gases is pushed back and forth in the volume reflector 108. This type of circle system comprising a volume reflector in which a drive gas acts directly on the gases exhaled from a patient is often referred to as a volume reflector system. It may also be called a Werner system after the inventor of U.S. Pat. No. 4,989,597, wherein such a system is described in more detail. The volume reflector or exchanger may also be called a Werner volume. Hereinafter, the terms volume reflector and volume reflector system will be used.
In FIG. 1B, the drive circuit 101B is a breathing bag 120 which is manually maneuvered by a medical professional. During exhalation, the exhalation gases flows through the patient connector 104, the expiration leg 110 and into the breathing bag 120. The medical professional then initiates inhalation by compressing the breathing bag 120, which compression re-supplies the exhalation gases to the patient via the inspiration leg 112, in which CO2 is removed and additional fresh gas added, and via the patient connector 104. The circle system 101B also comprises a pressure regulating valve 122 through which excess gases can leave the breathing circuit 100B. Typically, the excess valve 122 is an adjustable pressure limiting (APL) valve which serves to set a highest allowable pressure in the circle system 101A and hence limit the amount of pressure build-up that can occur during manual ventilation due to the compression of the breathing bag 120 and the supply of fresh gas via the fresh gas supply line 116. The APL valve 122 also serves to set a filling degree of the breathing bag 120. Just as the expiration valve 117 described above with reference made to FIG. 1A, the excess valve 122 is normally connected to a scavenging or recovery system to make sure that the excess gases leaving the circle system 101B do not harm attending medical personnel.
In FIG. 1C, the drive circuit 101C has a ventilator 106 and a resilient bellows 124 which is contained in a container 126. The ventilator 106, has an inspiratory portion 112 and an expiratory portion 118 arranged in fluid communication with the container 126 to pneumatically operate the resilient bellows 124 and thereby control the breathing of a patient 102 connected to the system 101C. The resilient bellows 124 in the container 124 is often referred to as a “bag-in-bottle”. The functionality of the circle system 101C is very similar to the functionality of the circle system 101B described above with reference made to FIG. 1B. They differ mainly in that the compressible reservoir that collects and re-supplies the exhalation gases is manually operated in circle system 101B (breathing bag 120) while it is automatically operated in circle system 101C (resilient bellows 124) by means of an automatic ventilator. The circle system 101B also comprises an excess valve 128, typically a pop-off valve 128, which is arranged to open if the resilient bellows or bag-in-bottle 124 hits the roof of the container 126.
It should be noted that most breathing apparatuses of today offers both a manual and an automatic ventilation mode. That is, most breathing apparatuses comprise a manual bag and, e.g., a bag-in-bottle arrangement allowing the medical professional to choose between manual or automatic ventilation by turning a control knob or the like on the breathing apparatus. The drive circuits 101-C have herein been illustrated in different drawings only to facilitate understanding of the functionality of the different breathing circuits 100A-C. Those skilled in the art appreciate how to re-design any of the circle systems 100A and 100C to allow a manual bag 120 to be used as a complementary drive circuit to any of the drive circuits 101A or 101C.
Circle systems of the above mentioned types all suffer from drawbacks when it comes to ventilating different patients having different lung capacities. All the illustrated prior-art circle systems 101A-C have some kind of collector for exhalation gases. In system 101A the collector is an open-ended collector in form of the volume reflector 108, in system 101B the collector is a manual breathing bag 120, and in system 101C the collector is a resilient bellows 124. If the collectors 108, 120, 124 have too small a volume compared to the gas volume expired by the ventilated patient at each breath, i.e. the tidal volume of the patient, some of the exhalation gases will leave the circle systems through the valves 117, 122, 128 which causes an undesired loss of exhalation gases one would like to re-supply to the patient. Not only does the loss of exhalation gases increase the consumption of, e.g., anesthetic gases, the increased fresh gas flow (FGF) required for compensating the losses also undesirably cools and desiccates the pulmonary system of the patient. However, if the volume of the collector 108, 120, 124 is too big, the excess collector volume will add to the volume of the breathing circuit connecting the collector 108, 120, 124 and the patient 102, and if the breathing circuit volume is big compared to the tidal volume of the patient, the responsiveness of the circle system 101A-C is decreased due to the large compressible volume in the circle system. It is easily understood that if, e.g., an infant patient having a tidal volume of 0.2 liters is connected to a circle system 101A-C having a breathing circuit volume of 2 liters, the large compressible volume will introduce system delays severely decreasing the accuracy in the control of the circle system. The poor control of the gas flows within the circle system so arising makes it hard to deliver a well-defined gas volume to the patient, and/or to deliver the gas at a well-defined pressure. The problem of too big collector volume is not as critical in the cases in which the collector is a resilient reservoir, such as the manual bag 120 or the resilient bellows 124, since their volume is adapted to the amount of gas filling them. However, it is highly important that the rest of the breathing circuit volume is small also in these cases.
In order to avoid some of the above problems associated with circle systems, it is known in the art to during manual ventilation use differently sized breathing bags when ventilating patients having different tidal volumes to ensure that the breathing bag can hold all the gas volume exhaled by the patient. It is, however, important not to use too big a bag in order to maintain a good feeling and “connection with the patient's lungs” when squeezing it. It is also known to decrease the volume of the patient connector 104 in order to decrease the volume of the breathing circuit connecting the collector 108, 120, 126 and the patient when ventilating infant or pediatric patients having small tidal volumes. For example, U.S. Pat. No. 5,393,675 discloses an anesthesia gas delivery system wherein the patient connector comprises a variable volume patient reservoir to decrease the volume of the patient connector when ventilating pediatric patients.
Most breathing circuit systems of today are modular breathing systems meaning that they comprise a plurality of separate modules that can be interconnected to constitute different types of breathing circuits. Such modular breathing systems typically comprise a drive circuit module, a patient circuit module and patient connectors. The drive circuit may be a mechanical ventilator, a manual bag or a bag-in-bottle arrangement, as described above with reference made to FIGS. 1A-C. The patient circuit module typically comprises the majority portion of the breathing circuit, non-return valves, carbon dioxide absorbers, additional filters, flow and pressure sensors, etc., and is often molded in one piece to be easily connected to, e.g., an anesthesia machine. Such a patient circuit module intended for interconnection between a drive circuit module and a patient connector is sometimes referred to as a patient cassette. U.S. Pat. No. 5,549,105 and EP 0919253B1 disclose examples of such patient cassettes, referred to as “closed circuit patient system” and “patient breathing circuit”, respectively.
The type and functionality of a modular breathing circuit is generally determined by the configuration of the patient circuit comprised in the patient cassette. By connecting different patient cassettes with different patient circuit configurations to a breathing apparatus, e.g. an anesthesia machine, breathing circuits optimized for different respiratory treatments, different drive circuits and/or different patient categories can be obtained.
This implies that if a care institution wants to be able to provide respiratory treatment to both children and adult patients and have the choice to choose what drive circuit to use and what breathing circuit type to use, the care institution must have a number of patient cassettes at hand. Typically, however, care institutions have a standard patient cassette adapted for an adult patient, a particular breathing circuit type and a particular drive circuit.
It is thus a need for a more flexible patient cassette that can be optimized to different types of drive circuits and different patients, and which can be optimized for use in different breathing circuit types.